Over half of the artificial light produced in the United States comes from lamps in which an electric discharge through a gas is used to produce illumination (J. Waymouth, Electric Discharge Lamps, MIT Press, Cambridge, Mass., 1971). The prevalence of electric discharge (e.g., fluorescent) illumination has led us to develop ways to inexpensively use discharge lamps for communication.
The basic idea of using lighting to send information as well as to provide illumination appears to have originated at least as early as 1975 (M. Dachs, "Optical Communication System," U.S. Pat. No. 3,900,404, August 1975). In the Dachs patent, the inventor discloses an analog amplitude-modulation (AM) scheme to modulate the arc current in a fluorescent lamp, the "carrier" signal, with an audio information signal. A more recent patent (K. King, R. Zawislak, and R. Vokoun, "Boost-Mode Energization and Modulation Circuit for an Arc Lamp," U.S. Pat. No. 5,550,434, August 1996) discloses an updated electronic circuit that also provides for AM modulation of the arc current with an analog signal. Such techniques are generally undesirable for the direct transmission of data since, among other reasons, low frequency content in the data may lead to perceptible flicker in the light output, and the noise immunity of the overall transceiver system is not optimal. Techniques for encoding digital information have been described in M. Smith, "Modulation and Coding for Transmission using Fluorescent Lamp Tubes," U.S. Pat. No. 5,657,145, August 1997 and T. Gray, "Transmission System," U.S. Pat. No. 5,635,915, June 1997 which employed either a pulsed AM or a phase modulation technique, respectively. Both techniques transmitted data at a rate that is on the same order of magnitude as that of the power-line frequency (50/60 Hz), i.e., relatively slowly compared to typical modem lamp arc frequencies in the range of 20,000 to 40,000 Hertz. Other communication schemes have also been proposed that do not use the lamp light as the carrier, but instead use the lamp fixture as an antenna for transmitting conventional radio wave or microwave signals. In K. Uehara and K. Kagoshima, "Transceiver for Wireless In-Building Communication Sytem [sic]," U.S. Pat. No. 5,424,859, June 1995, for example, the inventors disclose techniques for mounting a microwave antenna on the glass surface of fluorescent and incandescent lamps.
In T. Buffaloe, D. Jackson, S. Leeb, M. Schlecht, and R. Leeb, "Fiat Lux: A Fluorescent Lamp Transceiver," Applied Power Electronics Conference, Atlanta, Ga., June 1997, the authors outlined the possibility of using pulse-code modulation to transmit data with a fluorescent lamp. This scheme made use of a tri-level pulse coding, which led to a ballast design with a relatively high-complexity compared to the architectures described in the present invention. Also, the associated receiver was more complicated, and unable to support the high data rates achievable with the present invention.
We have invented a communication network based on frequency modulated radiation (e.g., visible light) that eliminates the disadvantages of the systems in the prior art. It enables higher power commercial scale lighting systems to be used to transmit the signal. It eliminates undesirable visual flicker in the system and so allows simultaneous continuous use of the lighting fixtures as lighting while also providing the medium for communication. It allows analog, digital or analog/digital data to be sent and received. It increases the bandwidth available to transmit data, and it enables a number of applications, such as multiple digital data streams, to be performed using a single lamp. Improvements made in the current invention can result in unprecedented performance advantages in the operation and implementation of lamp transceiver systems.
This invention is the first to propose establishing a transceiver system using any radiating transmitter with dual utility where the primary utility is any application, not just illumination but also possibly range finding, lane marking, or other applications, and the secondary utility is communication. This invention is the first to propose the transmission of bandlimited analog information such as audio signals by using frequency modulation, which enhances the noise immunity and available bandwidth over previous schemes while specifically avoiding sensory perceptible flicker in the transmission. It is the first to propose the efficient transmission of digital data using pulse code frequency modulation, and also the first to propose encoding digital bits in sidebands around the carrier frequency of the transmitter. It is the first to propose the use of a nonlinear detector in a dual-use network receiver to improve settling and detection time of pulse-coded data. These schemes for the transmission and reception of digital data substantially enhance the available data transmission rate in comparison to schemes in the prior art, again while elimination perceptible flicker. It is the first to disclose schemes for creating multiple data transmission channels using the same transmitter, and the first to propose a receiver in a "dual-use" network capable of selecting one channel from a spectrum of available choices. It is the first to propose a receiver with variable "lock-in" or transmitter capture characteristics, allowing the tailoring of the behavior of the receiver as it locks on to different transmitters. This feature could be especially important for optimizing the receiver's behavior in way-finding applications, and in environments with many different closely spaced transmitters, to ease the process of acquiring and holding a data channel between the transmitter and receiver.